Merge pull request #3230 from servo/cargoify

Cargoify servo
Update the README a bit.
Only pass extra link flags on OS X.

Fixes #3249
Merge pull request #3259 from metajack/update-readme

Update the README a bit.
Merge pull request #3260 from metajack/fix-cef-linux

Only pass extra link flags on OS X.
Refactor python/tidy.py to not use globals. Fixes #3242
Cleaned up python/licenseck.py

- Moved logic into tidy.py
- Removed explicit BSD license exceptions, they are already ignored by
  being inside the codegen directory.